B200F misfire...any ideas?

[AllHailKingVolvo]

My 744 has developed a misfire recently...It's particularly noticeable at lower revs when the engine is struggling a bit for torque.

I suspect overfuelling as the car smells of fuel from the outside whilst running, and when following it in my 240 (with my partner driving the 740) the exhaust fumes made my eyes water.

I recently changed the coolant temp sensor, which improved MPG, but admittedly I don't think it was a Bosch unit despite coming in a Bosch box. I'm also starting to wonder if the MAF is gradually failing. The idle of the car has always been a bit lumpy (as documented in another thread at great length!) But it's noticeably more pronounced now.

Any suggestions would be fantastic!

[dingov70]

Morning , you need to look for air leaks ( soapy water over joints / along rubber pipes ) , fuel leaks VERY obvious , dirty MAF or the best one & one I see a large number of Lambda sensors becoming slow reacting and failing to alter fuelling ratio's quickly enough .

A live data read with diagnostic scanner ( Snap on or similar ) should allow you to read the values from the ecu with the engine running & google corect values to compare if necessary . A friendly MOT tester should be able to offer an emession test and if your CO readings fluctuate wildly / suddenly , then you are pretty much on the money

[Laird Scooby]

First thought would be the FPR or Lambda sensor Phil. A failed Lambda will give a reduced output fooling the ECU into thinking it's running lean so the ECU will add fuel, sending the mixture very rich. I had a Lambda sensor fail literally during the MoT emissions test and i watched the CO% go from 0.1 to 9.8% at which point the tester pulled the pipe out of the exhaust! All in the matter of a few minutes!

Checking the FPR is easier - get the engine idling and remove the vacuum pipe from it, putting your thumb over the open vacuum hose. Watch the stub on the FPR, if petrol comes out, whether a drop, dribble or drowning amount, the FPR is FUBAR - new ones are available from Rock Auto for about £40 including S&H, currency exchange fee and Import VAT (if i've got my maths right) :

https://www.rockauto.com/en/moreinfo...288458&jsn=408

Check the part numbers, they only got the 2.3 in the USA but i'm 99% certain the same FPR was used on both the B200F and B230F. Doubtless you could get it in the UK as well but the price is likely to be a lot higher!

[kelvinp]

Ah, this is interesting.

I've had a couple of odd things with my 740 recently that I've not been able to explain, there have been a couple of occasions where the engine has stalled or tried to stall while idling and I've been sure that there was a slight smell of petrol. It took quite a few turns before it would start again.

The idle does sometimes seem a little uneven and can just see a slight flicker on the rev counter.

Today, I went to pull out from junction and putting my foot down it went as if to cut out and I gave it a good run down the motor way and on a steady hill climb it did feel as though it was slightly missing or perhaps I was imagining it.

I've checked the LH2.4 diagnostics and not showing anything on it.

[Laird Scooby]

Originally Posted by kelvinp

Ah, this is interesting.

I've had a couple of odd things with my 740 recently that I've not been able to explain, there have been a couple of occasions where the engine has stalled or tried to stall while idling and I've been sure that there was a slight smell of petrol. It took quite a few turns before it would start again.

The idle does sometimes seem a little uneven and can just see a slight flicker on the rev counter.

Today, I went to pull out from junction and putting my foot down it went as if to cut out and I gave it a good run down the motor way and on a steady hill climb it did feel as though it was slightly missing or perhaps I was imagining it.

I've checked the LH2.4 diagnostics and not showing anything on it.

Check out the FPR as outlined in my post above - it sounds exactly the same as what mine was doing (B280E but LH2.2 - same as the LH 2.4 but without the Lambda sensor) and the new FPR cured it. Also mine would occasionally hunt on idle or when using the creep function of the auto box for maneouvering.

The LH2.2/2.4 can't be interrogated easily by a fault code reader, it's possible but it won't show fuel pressure fluctuations because there isn't a sensor for it!

[AllHailKingVolvo]

Thanks guys! Will check the FPR in the morning as the first port of call. I have some contact cleaner so will clean the MAF too.

The car has always had a bit of a lumpy, hunty idle, but lots of work I did managed to quiet that down and smooth it out somewhat. It has always had a very slight miss and seemed a little down on power on inclines but it's markedly worse now, which would seem to point to a failing component...I'll do some sleuthing!

[Laird Scooby]

Originally Posted by AllHailKingVolvo

Thanks guys! Will check the FPR in the morning as the first port of call. I have some contact cleaner so will clean the MAF too.

Use carb cleaner Phil, not contact cleaner in the MAF - simple reason is carb cleaner is pure solvent, contact cleaner contains an oil to lubricate the switch contacts. That oil will stay on the hot wire and eventually carbonize, causing insulation so the MAF won't see the temperature changes and cause hesitancy and/or over fueling when it does respond.

With everything you've said so far, i'd be looking at the FPR first, sometimes they don't leak and just go high pressure or as mine did, low pressure but if you pull the vacuum hose off with it running, you should hear something change in the engine note - if you don't (it's usually very slight) there's a good chance it's past its sell-by date!

[Jimsiss]

Some carb cleaners have a bit of oil in them to lubricate so just check. I’ve used one with oil in before to clean a few sensors and not had a problem but it’s not ideal.

[Laird Scooby]

Originally Posted by Jimsiss

Some carb cleaners have a bit of oil in them to lubricate so just check. I’ve used one with oil in before to clean a few sensors and not had a problem but it’s not ideal.

That's interesting - something i've not seen before but i'll certainly check from now on!

Adding oil to carb cleaner kind of defeats the object of it being a cleaner, it's meant to remove all the oil and grease and gum/residue from the carb and air intake so you have something clean to start with.

Switch cleaner on the other hand does need it as once it's cleaned the residue/dirt etc from the contacts, they need some lube or the contacts would wear prematurely.

[TonyS9]

Could be a weak spark. Overfueling shouldn't be a problem normally, just wasteful (all petrol engines over-fuel most of the time).

Maybe 1 plug is having problems, or general dirt + damp.

Inspect and clean. If you can run it in the dark and look for corona leakage.